In an image forming apparatus such as a digital color copying machine, for example, in which different color images are overlaid on one recording material, a positional deviation, that is, a color misregistration, is a problem. In order to correct the positional deviation (color misregistration), it is important to detect, with high precision by an optical sensor, a color misregistration correction pattern (toner image for correction) in the form of rectangular images formed on an image bearing member by image forming stations.
However, simply forming the rectangular color misregistration correction pattern results in deterioration of a detection accuracy attributable to edge effect. In view of this, a means is proposed in which line patterns are formed before and after the color misregistration correction pattern with spaces so that an increase of a toner image density at the end portions of the color misregistration correction pattern attributable to the edge effect is suppressed (Japanese Laid-open Patent Application 2006-189625).
Here, with the structure disclosed in the Japanese publication, the developer used there is a developer comprising magnetic toner. On the other hand, in the case of using developer containing non-magnetic toner and magnetic carrier particles, there is a likelihood that a density at an end portion of the color misregistration correction pattern may decrease.
That is, in the developing device using the two component developer, magnetic chains including the toner and the carrier are formed along magnetic flux lines provided by a magnet roller disposed in a developing sleeve, and the toner is supplied to the position of an electrostatic latent image on the photosensitive drum from the magnetic chains, thus developing the electrostatic latent image. At this time, a so-called counter charge phenomenon occurs in which the toner (negative charging) once transferred onto the photosensitive drum from the magnetic chain (positive charging) formed on the developing sleeve by the carrier returns to the magnetic chain. Generally, a peripheral speed of the developing sleeve is made higher than the peripheral speed of the photosensitive drum to enhance the development property.
In such a case, at a rear end of the electrostatic latent image (with respect to a sub-scan direction) which is a boundary between an image formation region and a non-image formation region, the magnetic chain having passed an electric field region with which the toner does not develop the image passes. In this region, the toner is returned to the magnetic chain with the result of the decrease of the amount of the toner on the photosensitive drum, that is, the toner amount decreases at the rear end of the toner image.
In this manner, when the sensor detects the rear end of the color misregistration correction pattern having the decreased density, the detected position of the color misregistration correction pattern involves an error, with the possible result of incapability of satisfactory color misregistration correction (correction of the position of the toner image formed by the image forming station).